Methods of reducing floating body effect

ABSTRACT

Methods of reducing the floating body effect in vertical transistors are disclosed. The floating body effect occurs when an active region in a pillar is cut off from the substrate by a depletion region and the accompanying electrostatic potential created. In a preferred embodiment, a word line is recessed into the substrate to tie the upper active region to the substrate. The resulting memory cells are preferably used in dynamic random access memory (DRAM) devices.

REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.11/437,999, filed May 18, 2006, which is a divisional of U.S. patentapplication Ser. No. 11/010,752, filed Dec. 13, 2004, the disclosures ofwhich are hereby incorporated by reference in their entireties herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of integrated circuitfabrication, specifically to the formation of transistors.

2. Description of the Related Art

Since the introduction of the digital computer, electronic storagedevices have been a vital resource for the retention of data.Conventional semiconductor electronic storage devices, such as DynamicRandom Access Memory (DRAM), typically incorporate capacitor andtransistor structures in which the capacitors temporarily store databased on the charged state of the capacitor structure. In general, thistype of semiconductor Random Access Memory (RAM) often requires denselypacked capacitor structures that are easily accessible for electricalinterconnection.

In order to increase efficiency of memory devices, there is an effort tocreate smaller memory cells. DRAM memory cells can shrink by decreasingthe minimum feature size (F) through new and advanced lithography andetching techniques. Memory cells can also be decreased by designing amemory cell that requires less chip real estate in terms of minimumfeature size. For example, many DRAM devices on the market today have amemory cell size of 8F² or greater. However, DRAM devices can be madeeven smaller, such as 6F² or 4F². An example of a 6F² device isdisclosed in U.S. Pat. No. 6,734,482, issued to Tran, et al, thedisclosure of which is incorporated herein by reference.

One method of designing smaller memory cells is to use verticaltransistors, particularly vertical surrounding gate transistors (VSGT).VSGTs are typically metal-oxide-semiconductor field effect transistors(MOSFET) and can be designed in several different manners. In manyembodiments of VSGTs, an upper active region of the device can beelectrically separated from substrate by the lower source/drain regionand/or by a dielectric layer. Such an electrically separated activeregion is known as a “floating body.” The floating body can charge upand cause a transient bipolar current. This current can cause memoryfailures through cell capacitor discharge. Without being limited bytheory, one explanation is that a transient hole current is caused bycarriers generated during the high state. It is therefore desirable tominimize the floating body effect for small memory cells.

SUMMARY OF THE INVENTION

In an aspect of the invention, a method of reducing the floating bodyeffect in a memory cell is disclosed. The method comprises forming anisolation trench within a substrate. A ridge is also defined in thesubstrate proximate to the isolation trench. The isolation trench isfilled with an insulating material. A silicon pillar with an upperactive area is formed over the ridge after filling the isolation trench.A source region is formed on the ridge adjacent to the silicon pillar.The insulating material in the isolation trench is recessed below a toppoint of the ridge in the substrate. A word line is deposited afterrecessing the insulating material, wherein the word line extends lowerthan the top point of the ridge. The method further comprises forming acapacitor over the silicon pillar after depositing the word line.

In another aspect of the invention, a memory array for a DRAM isdisclosed. The array comprises a plurality of source regions on aplurality of substrate ridges. A plurality of isolation trenches arepositioned between substrate ridges. A plurality of silicon pillars arepositioned in rows on the substrate ridges between source regions. Aplurality of word lines each surround one row of silicon pillars,wherein the word lines are substantially recessed to a depth below a toppoint of the substrate ridges. A plurality of digit lines are positionedabove the silicon pillars and a cell capacitor overlies each siliconpillar.

A memory cell is disclosed in another aspect of the invention. Thememory cell comprises a vertical transistor including a source regionwithin a ridge of semiconductor material, an upper active area on asilicon pillar extending above the ridge, and a gate surrounding thesilicon pillar. The gate is recessed into a trench next to the ridge. Acell capacitor contact plug is directly over the silicon pillar. Thecell further comprises a cell capacitor including a lower electrode overand electrically connected to the cell capacitor contact plug, acapacitor dielectric over the lower electrode, and an upper electrodeover the capacitor dielectric.

In another aspect of the invention, a method of forming a memory arrayis disclosed. The method comprises defining a plurality of paralleltrenches and ridges in a p-doped bulk silicon substrate. The trenchesare filled with an isolation material. A mask is formed over thesubstrate to create windows on the ridges in which to form pillars. Aplurality of pillars is created over the ridges after forming the mask.The isolation material is recessed in the trench after forming thepillars. A plurality of word lines are deposited over the isolationoxide and the ridges and surrounding a row of pillars after forming thepillars. The word lines descend substantially into the trenches on twosides of each pillar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic plan view of a memory array at a first stage ofprocessing.

FIG. 1B is a schematic cross-section of the array of FIG. 1A along line1B-1B of FIG. 1A.

FIG. 1C is a schematic cross-section of the array of FIG. 1A along line1C-1C of FIG. 1A.

FIG. 2A is a schematic plan view of a memory array after formation ofsilicon pillars for the array.

FIG. 2B is a schematic cross-section of the array of FIG. 2A along line2B-2B of FIG. 2A.

FIG. 2C is a schematic cross-section of the array of FIG. 2A along line2C-2C of FIG. 2A.

FIG. 3A is a schematic plan view of a memory array after formation ofword lines for the array.

FIG. 3B is a schematic cross-section of the array of FIG. 3A along line3B-3B of FIG. 3A.

FIG. 3C is a schematic cross-section of the array of FIG. 3A along line3C-3C of FIG. 3A.

FIG. 4A is a schematic plan view of a memory array after planarizationof vertical transistors of the array.

FIG. 4B is a schematic cross-section of the array of FIG. 4A along line4B-4B of FIG. 4A.

FIG. 4C is a schematic cross-section of the array of FIG. 4A along line4C-4C of FIG. 4A.

FIG. 4D is a schematic plan view of the memory array of FIG. 4A afterformation of source regions.

FIG. 5A is a schematic plan view of a memory array after formation ofdigit lines for the array.

FIG. 5B is a schematic cross-section of the array of FIG. 5A along line5B-5B of FIG. 5A.

FIG. 5C is a schematic cross-section of the array of FIG. 5A along line5C-5C of FIG. 5A.

FIG. 6A is a schematic cross-section of the array of FIG. 5A afterformation of cell capacitors to complete the memory cells.

FIG. 6B is a schematic three dimensional view of the array of FIG. 5Aafter formation of cell capacitors to complete the memory cells.

FIG. 7 is a schematic cross-section illustrating an alternativeembodiment to FIG. 3B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the context of this document, the term “semiconductor substrate” isdefined to mean any construction comprising semiconductor materials,including, but not limited to, bulk semiconductor materials such as asemiconductor wafer (either alone or in assemblies comprising othermaterials thereon), and semiconductor material layers (either alone orin assemblies comprising other materials). The term “substrate” refersto any supporting substrate, including, but not limited to, thesemiconductor substrates described above. Also in the context of thisdocument, the term “layer” encompasses both the singular and the pluralunless otherwise indicated.

In small (e.g. 4F²) memory cell designs, a vertical transistor can beused. These transistors typically use silicon pillars to form at least aportion of the active region. Silicon pillars can be formed usingmultiple masks and providing a spacer lining sidewalls of small windowsbefore selectively growing epitaxial silicon within the windows. Atransistor formed using such a selective epitaxial growth (SEG) processis described in U.S. Pat. No. 6,756,625, issued to Brown (hereinafterBrown), the disclosure of which is hereby incorporated by reference. TheBrown patent discloses a memory cell with a digit line beneath thesilicon pillar.

The memory cell in that patent has several benefits. First, the size ofthe memory cell is 2F×2F, or 4F², meaning that each side of the memorycell measures only two times the minimum feature size (F) that can beprovided for using the lithography processes for the process node.Additionally, the Brown process used to form the epitaxial postsprovides a consistent and efficient method of forming epitaxial siliconposts to be used for vertical transistors.

However, the floating body effect, which can cause a cell capacitor todischarge and cause memory faults, commonly exists in memory cells thatuse vertical transistors. As described above, the floating body effectoccurs when an active region on the pillar, typically the drain, becomesisolated from the substrate. However, the skilled artisan willunderstand that the source could also be positioned at the top of thetransistor pillar. The drain can be isolated by a dielectric layer or bythe depletion region of a lower active area. When this occurs, the upperactive area can charge up and cause the cell capacitor to discharge whenit is supposed to remain in a charged state (i.e., when the transistoris ostensibly “off”). Ultimately, this will cause memory faults in thearray.

One method of reducing the floating body effect involves the use of agate extending below an area that might cut off the active region on thetop of the pillar from the substrate. An example of this is extendingthe gate below a depletion region to tie the upper active area to thesubstrate. In an article by B. Goebel, et al., presented at the 2002International Electron Devices Meeting (IEDM) and printed in the 2002International Electron Devices Meeting Technical Digest, pp. 275-278,the idea of such a gate was proposed. That article is herebyincorporated herein by reference.

However, the scheme proposed by the Goebel et al. article is difficultto integrate, particularly for memory cell designs of differentgeometries, such as cells with capacitors that are above the siliconpillars, or active areas extending into the substrate.

In a preferred embodiment, a word line is formed surrounding theepitaxially grown silicon pillars and recessed into the field oxide suchthat the lowest point of the word line is beneath the highest point ofthe source region. Using this structure, the word line creates a p-typeregion in the silicon pillar that it surrounds. This creates a channelin which charge can move from the drain down to the substrate.

Forming Memory Cells

In a preferred embodiment seen in FIGS. 1A-1C, a p-well 190 is formedwithin a semiconductor substrate 204 before forming masks 208, 209 (FIG.1A) on the substrate. The formation of the masks 208, 209 is discussedbelow. In a preferred embodiment, the p-well 190 extends from thesurface of the substrate to a depth of between about 5,000 Å and 10,000Å, more preferably between about 8,000 Å and 9,000 Å. Skilledpractitioners will appreciate that there are several ways to form thep-well region.

Also preferably prior to depositing the first mask 208 (nitride in theillustrated embodiment) and the second mask 209 (oxide in theillustrated embodiment), isolation trenches 202 (FIG. 1B) and substrateridges 203 (FIG. 1B) are defined within the substrate. The isolationtrenches 202 and the upper substrate ridges 203 preferably runvertically in FIG. 1A, (i.e. parallel to line 1C-1C). The trenches canbe formed by forming a mask over the semiconductor substrate 204 andetching the substrate 204. Preferably, the substrate in the array isetched to a depth of between about 2000 Å and 4000 Å, more preferablybetween about 2500 Å and 3500 Å. As seen in FIG. 1B, the isolationtrench 202 is filled with a field isolation material, such as fieldoxide 205. Preferably, the field oxide 205 is formed primarily using aspin-on deposition (SOD) oxide. The field oxide 205 may have additionallayers. For example, the field oxide 205 may comprise, starting from thetrench walls, a thin thermal oxide layer (e.g., less than about 100 Å),a thin nitride layer (e.g., less than about 100 Å), a TEOS oxide layer(e.g., less than about 200 Å), and a thick spin-on oxide to fill theremainder of the trench (e.g., about 5,000 Å).

FIG. 1C illustrates the first mask 208 and the second mask 209 over thep-well 190. In a preferred embodiment, the first mask 208 is siliconnitride, and the second mask 209 is silicon oxide. Preferably a padoxide is formed on the surface of the substrate before the first mask208 is formed. Once the first mask 208 is formed, a liner 207 is formedover the substrate and the first mask. In a preferred embodiment, theliner 207 comprises the same material as the first mask. The first maskpreferably has a thickness of between about 2000 Å and 3000 Å. Theregions that will form part of the transistor active areas are beneaththe first mask 208. The active areas can be doped prior to formation ofthe masks, or during subsequent processing.

In a preferred embodiment, vertical transistors are formed usingepitaxial silicon posts, which define part of the active area andinclude a vertical surround gate (VSG) channel. The epitaxial posts areformed in a similar fashion as the posts in the Brown patent, which isincorporated herein above. FIG. 1A is a plan view of an array beforeformation of the epitaxial posts. Two mask patterns are shown over asubstrate 204. The first mask 208, preferably nitride, is deposited andpatterned to form lines over the substrate. The liner 207 is formed inthe gaps between lines of the first mask before the second mask 209 isformed between the lines of the first mask 208. Preferably, the secondmask 209 is an oxide such as a tetraethyl orthosilicate (TEOS) oxide ora spin-on deposition (SOD) oxide. The alternating structure of the masks208, 209 can be seen in FIG. 1C, a schematic cross section along line1C-1C.

In FIG. 2A, the second mask 209 has been patterned. In a preferredembodiment, the second mask is patterned by forming a perpendicular setof mask lines (not shown) and selectively etching to leave alternatingblocks of mask and windows of exposed substrate. FIG. 2B is a schematiccross-section along line 2B-2B, and illustrates a spacer 210 lining thesidewalls of the window region above the exposed substrate. Preferablythe spacer 210 is an oxide such as a TEOS oxide. A blanket layer ofoxide is deposited over the substrate 204 and into the windows. A spaceretch is then performed to etch the horizontal surfaces of the oxide,forming the spacer 210 and exposing a smaller window of the substrate204. In one embodiment, a thin nitride layer (not shown) can be formedover the spacer 210. The nitride layer is then spacer etched to allowfor a hydrofluoric acid wet pre-clean step prior to epitaxialdeposition. Epitaxial silicon 220 is then selectively grown in thenarrowed window on the substrate ridge 203. In a preferred embodiment,the epitaxial post 220 has a shape that is approximately rectangular orsquare. However, the shape of the epitaxial post 220 can also be acylinder, or other geometric shapes.

As seen in FIG. 2B, a cross-section along line 2B-2B, the patterning ofthe second mask 209 forms islands upon the substrate 204. The islands ofmask 209 and the lines of the first mask 208 define the windows on theportions of the ridges 203 over which the narrower epitaxial posts 220are formed.

The silicon pillars 220 are formed using selective epitaxial growth(SEG) techniques known in the art. Preferably, the silicon pillars 220are formed as undoped epitaxial silicon. A p-type dopant, such as boron,is preferably implanted in the pillars after the epitaxial growth. Afterthe SEG step, the pillars 220 are recessed to a preferred height abovethe substrate 204. Preferably, the silicon pillars 220 have a height ofbetween about 1200 Å and 2000 Å, more preferably between about 1400 Åand 1800 Å. Skilled practitioners will appreciate that the SEG step canuse a variety of parameters to form the silicon pillars 220. In oneembodiment, the drain region 221, which is preferably situated on thetop of the pillar 220, can be formed by implanting an n-dopant in thetop of the pillar. The upper active region 221 on the silicon pillar 220preferably serves as the drain of the transistor. The drain regions 221can be formed by implanting an n-dopant into the silicon pillars 220,either immediately after the SEG step, or at a later stage ofprocessing. The drain regions 221 could also be formed by in situ dopingduring latter stages of SEG deposition after forming the channel or bodyof the pillar 220 as lightly p-doped silicon. Skilled practitioners willappreciate that the drain region can be doped at any of severaldifferent stages and by any of various different methods.

In FIGS. 3A-3C, the second mask 209 and the spacers 210 have beenremoved. The silicon pillars 220 can be seen in columns atop the ridges203 in the substrate 204. The field oxide 205 is recessed within theisolation trench either during or after the process of removing thesecond mask 209 and spacers 210. In a preferred embodiment, a bufferedoxide etch (BOE) solution will be used to etch the oxide second mask 209and spacers 210. The nitride liner 207 (FIG. 2B) acts as an etch stopfor the BOE etch process. A short wet nitride strip is then performed toremove the nitride liner 207. In order to reduce the floating bodyeffect, an oxide dry etch, preferably selective to the first mask 208and the epitaxial silicon 220, is used to recess the field oxide 205 toa selected level. As discussed below, the selection of how far to recessthe field oxide is based on both the floating body effect and gatecapacitance. By recessing the field oxide 205, the gate 240 can extendbeneath the height of the silicon ridge 203. While the timing of therecessing of the field oxide is described as being after the formationof the pillars 220, the field oxide 205 can also be recessed at otherprocessing stages. For example, the field oxide 205 can be recessedafter the deposition of the first mask 208 and before the formation ofthe nitride layer 207.

A conformal dielectric layer is formed over exposed silicon, includingthe exposed portion of the substrate ridges 203 and the silicon pillars220. This dielectric layer will serve as the gate dielectric 230 (FIGS.3B and 3C) over the pillars 220. In the illustrated embodiment, athermally grown gate oxide 230 formed over the substrate ridge 203 willisolate the pillars 220 from the word lines 240. The gate oxide 230 ispreferably formed by thermal oxidation or an oxide, such as a highdielectric constant oxide, could be deposited. In a preferredembodiment, the gate dielectric 230 is formed to be quite thin.Preferably, the gate dielectric 230 is between about 10 Å and 100 Å,more preferably between about 60 Å and 80 Å.

The word lines 240 are deposited surrounding the gate dielectric 230 andover the field oxide 205. Preferred materials for the word line includedoped poly or a fully silicided conductor, such as CoSi₂ or NiSi.Preferably a blanket layer of word line material is deposited over thearray. The word line material is recessed back to form the word lines240, which act as gates for the vertical transistors. After patterningthe word lines 240, each word line 240 will surround a row of pillars220 running in a direction perpendicular to the trenches 202 and ridges203. By extending the gates/word lines 240 below the original height ofthe substrate 204, the problem of the floating body effect isalleviated. The electrical path from drain regions 221 to the substrate204 is kept intact by the presence of the word line material 240. Thus,there are available carriers in the pillar to allow charge to move downthe pillar 220 when the transistor is “off”. Drain regions 221, situatedat the top of the pillars 220, are thus “tied” to the substrate due tothe lower extension of the gates 240.

However, by dropping the gates 240 into the isolation trenches 202, gatecapacitance is increased. Therefore, the depth to which the field oxide205 is recessed and the word lines 240 extend into the trenches 202 is abalance between competing interests: the reduction of the floating bodyeffect and the gate capacitance. In a preferred embodiment, the fieldoxide is recessed (and the word line 240 or gate extends downwardly)relative to the upper surface of the substrate ridge 203 by betweenabout 0 Å and 1500 Å, more preferably between about 500 Å and 1000 Å.Depending on the application and the concerns of gate capacitance andfloating body effect particular to the application, the depth to whichthe field oxide 205 is recessed can be varied.

The depletion regions that can cut off the pathway from the upper activeregion 221 to the substrate 204 can be measured in several ways. Acommon way to measure the depletion region is by measuring the midgappotential, the electrostatic potential in the substrate 204 at thecenter 243 of the ridge 203 between source regions. Because of the shapeand location of the source regions 206 (namely on either side of but notbeneath the pillars 220), they can be seen in FIG. 3C, but not FIG. 3B.The ridge 203 beneath and extending away from each pillar 220 forms apart of the transistor channel. Using the recessed word line 240, theelectrostatic potential is reduced directly beneath the drain 221.Without using the recessed gate, depletion regions in the substrate canmeet. For example, without using the recessed word line 240, the midgappotential was simulated to be approximately 1.5 eV. However by using therecessed gate, the midgap potential falls in simulations toapproximately 0.1 eV. Preferably, the use of the recessed word line 240causes a reduction in midgap potential of greater than about 75%, morepreferably greater than 85%.

FIGS. 4A-4C show vertical transistors after the deposition of aninsulation layer 256 and a planarization of the upper surface down tothe top of the first mask 208. Because the first mask 208 isapproximately 1000 Å taller than the pillar, some of the insulationlayer 256 will remain over the pillars 220. In a preferred embodiment,the insulation layer 256 is an oxide, such as undoped silicon glass(USG), borosilicate glass (BSG), phosphosilicate glass (PSG), orborophosphosilicate glass (BPSG). The upper surface is then planarized,preferably using a stop on nitride chemical mechanical polish (CMP)step. By using a stop-on-nitride CMP step, a sufficient insulation layer256 will remain after the CMP step.

In a preferred embodiment, the source regions may be formed after theplanarization of the insulation layer 256 and before the formation ofthe digit lines. In FIG. 4D, a mask 257 is formed over the array of FIG.4A. Preferably the mask 257 is parallel to the substrate ridges 203. Themask 257 preferably covers half of the substrate ridge 203 and is overhalf of the field oxide 205. The mask 257 is used to remove a portion ofthe first mask 208. Once the first mask 208 is patterned and etched, aportion of the substrate ridge 203 and the field oxide 205 will beexposed. The exposed region of the substrate ridge 203 can be implantedwith a dopant in order to form the source region 206. In a preferredembodiment, an n-type dopant is implanted. The implantation preferablydoes not substantially affect the properties of the exposed portion ofthe field oxide 205.

FIGS. 5A-5C illustrate the formation of digit lines that will be buriedby an interlayer dielectric, in which the cell capacitor is formed. InFIG. 5B, a schematic cross-section is seen through a row of pillars 220.In a preferred embodiment, the active areas of the transistor, i.e. thesource and the drain, are formed before formation of the digit lines.The drain regions 221 may be formed at several stages of processing whenthe top of the pillar 220 is exposed. The source regions 206, theformation of which is discussed above, are seen in FIG. 5C. The digitline 315, which is over a digit line contact 310, is seen over the fieldoxide 205. Preferably, the digit line 315 runs perpendicular to andabove the word line 240.

FIG. 5C illustrates the formation of a digit line plug 305 to connectthe digit line 315 to the source region 206 of the transistor. Byexposing the source region 206 as discussed with reference to FIG. 4Dabove, the fabrication of a contact plug 305 connecting the sourceregion 206 and the digit line 315 is facilitated. In a preferredembodiment, the digit line plug 305 is offset from the isolation trench202 by F/2 (i.e. half of the minimum printable feature size).Preferably, the digit line plug 305 is a conductive material, morepreferably a polysilicon layer, most preferably n-doped poly.

The digit line plug 305 contacts the lower portion of the digit line310. In a preferred embodiment, the digit line is multi-layered. Thelower portion of the digit line 310 and the upper portion of the digitline 315 are offset by F/2 as well. This allows the digit line 315 to beformed over the isolation trench 205. Forming the digit line over thetrench 205 can prevent interference caused by the digit line 310 and315.

In a preferred embodiment the lower portion of the digit line 310 ismetallic, such as a conductive nitride like titanium nitride. Morepreferably the lower portion of the digit line 310 is metal modetitanium nitride, a TiN layer that has a high titanium concentration andis particularly conductive. Skilled artisans will appreciate that thereare several methods of forming such a layer, including chemical vapordeposition (CVD) and atomic layer deposition (ALD). Preferably, theupper portion of the digit line 315 is a conductor, more preferably ahighly conductive metal, most preferably tungsten. An insulating liner320 is preferably deposited around the digit line 310 and 315. Theinsulating liner 320 can also be deposited in a recess in the digit lineplug 305. An insulation layer 323 is deposited over the upper portion ofthe digit line 315. Preferably the insulation layer 323 is a siliconnitride layer.

Preferably the digit line layers 310 and 315, and the insulation layer323 are formed by depositing blanket layers over the partiallyfabricated array of FIGS. 4A-4C. A photoresist mask is preferably formedin order to pattern these layers and an anisotropic etch process is usedto etch them. The nitride liner 320 can then be deposited over the edgesof the digit line layers 310 and 315, and the insulation layer 323. Anitride punch or spacer etch is preferably formed to contact the siliconposts 220. If the drain regions 221 have not been doped, they can bedoped after the spacer etch is performed to expose the silicon posts220. An oxide layer 325 is preferably deposited over the array before aplanarization process, preferably a CMP step.

After the digit line 315 and its associated layers are formed, a cellcapacitor is formed over the vertical transistor to complete the memorycell. In a preferred embodiment, a container capacitor is formed overthe vertical transistor.

Referring now to FIG. 6A, a cross section of two memory cells, the oxidelayer 325 is removed over the silicon posts 320 and a cell contact plug350 is formed. Preferably, the plug 350 is a conductor, more preferablythe plug 350 is a polysilicon layer. A structural layer 360 is formedover the insulating layer and the plug 350. Preferably the structurallayer 360 is an oxide, such as BSG, PSG, or BPSG. The structural layeris masked and etched to form a recess over and exposing each plug 350 inthe array.

A lower capacitor electrode 365 is conformally formed within the recessand over the structural layer. The lower electrodes 365 are preferablyetched or polished back to isolate each electrode from its neighboringcells. Preferred materials for the lower electrodes 365 include metals,such as tungsten, metal compounds, such as titanium nitride, andpolysilicon. A blanket layer of a capacitor dielectric 370 is thenconformally formed over the array. The capacitor dielectric 370 does notneed to be etched to be isolated from neighboring cells. Rather, in apreferred embodiment, the capacitor dielectric 370 can be one layer forthe entire array. Preferred materials for the capacitor dielectric 370include metal oxides, such as tantalum oxide, aluminum oxide, hafniumoxide, and zirconium oxide.

Finally, an upper electrode 375 is formed over the array. The materialfor the upper electrode 375 is preferably blanket deposited over thearray. Like the capacitor dielectric 370, the upper electrode does notneed to be isolated from neighboring cells; a single electrode layer 375can serve as the upper or reference electrode for multiple capacitors orthe whole array. Preferred materials include metals, such as tungsten,and metal compounds, such as titanium nitride and tungsten silicide(WSi_(x)). The upper electrode 375 can also comprise multiple films. Forexample, the upper electrode 375 can comprise a layer of titaniumnitride, a layer of polysilicon, and a layer of tungsten silicide. FIG.6B shows a schematic three dimensional view of a cell. The word line 240can be seen descending below the ridge 203 into the trench 202.

In another embodiment, the structural layer 360 can be removed prior todepositing the capacitor dielectric 370. This would allow both theinside and the outside of the lower container electrode 365 to be usedto increase the capacitance. The capacitor dielectric 370 and the upperelectrode 375 would then be formed conformally over the lower electrode365 and over the insulating layer 323. An insulator is deposited overthe array before the array is planarized, preferably using a CMP step.

Another embodiment is illustrated in FIG. 7, where the array is seen atthe stage of processing corresponding to FIG. 3B of the firstembodiment. In this embodiment, a portion 242 of the substrate ridge 203beneath the pillar 220 is implanted with an n-type dopant before theword lines 240 are formed. This extends the source regions 206 tooverlap with the pillars 220. This extended source 242 can be formeddirectly beneath the pillar 220, or it can stop at the edge of thepillar 220. This allows the depletion field to spread further down thesilicon post 220 rather than through the whole width of the siliconpost. The doped portion 242 is preferably formed before the word line isformed and after the active areas and the isolation trenches are formed.In a preferred embodiment, the doped portion 242 is used in combinationwith the word line 240 dropped into the field oxide 205.

Structure

Two memory cells are seen in FIG. 6A. N-doped source regions 206 areformed in the array and can be seen FIG. 5B and serve as the source forthe transistors. Isolation trenches 202 are formed within a substrate204. A silicon pillar 220 extends over a plateau or ridge 203 in thesubstrate 204, so that it is bordered on two sides by a ledge 262because the pillar 220 is narrower than the width of the substrate ridge203. Preferably, the other two sides of the pillar 220 are facing thelength of the substrate ridge 203. The source region 206 is formed inthe substrate ridge 203. The silicon pillar 220 and portions of theridge 203 are lined by a gate dielectric 230. A word line 240, which isdropped into the isolation trench 202 surrounds a row of silicon pillars220. The word line 240 follows the gate dielectric 230 around the pillarand down along the sides of ledge 262 into the isolation trench 202.

An isolation material 256 separates the word line 240 from the digitline conductors 310 and 315, which is connected to the active area 206through a digit line plug (not pictured). The pillars 220 can also varyfrom the embodiments described herein. For example, the pillar 220 canbe etched from the substrate, the pillar could take other shapes, or thedoping could be opposite (i.e. n-doped versus p-doped) from thosediscussed above. Also, the source region 206 may be extended to overlapwith the pillar 220 by doping an additional portion of the ridge 203.

A capacitor contact plug is connected to the silicon post 220 and to thelower capacitor electrode 365 The lower electrode is formed within astructural layer 360. A capacitor dielectric 370 is formed over thestructural layer 360 and within the lower electrode 365. An upperelectrode layer 375 is formed over the capacitor dielectric 370. Aninsulator can be deposited over the array to isolate the array duringsubsequent processing.

FIG. 6B is a schematic three dimensional view of the cell, which omitsseveral parts of the cell in order to clearly show the remaining parts.Preferably, a drain 221 is situated on the top of the pillar 220, and asource 206 is situated within the silicon ridge slightly offset from thepillar 220. While a capacitor plug (350 in FIG. 6A) is used in preferredembodiment, it is not shown over the drain in FIG. 6B in order to showthe relative position of the capacitor 330 to the digit line layers 310and 315. Additionally, the layer 310 is shown only with the plug 305underneath it. However, the digit line layers 310 and 315 are alsosupported over the substrate 204 by the oxide layer 256 (FIG. 6A) andseveral other layers which are not shown in FIG. 6B. The source contactsthe digit line plug 305 in order to form an electrical connection withthe digit line. An insulator layer 323 is over the digit line. Thecapacitor 330, positioned over the drain 221, is shown onlyschematically in FIG. 6B, but is preferably a container capacitor asdescribed in reference to FIG. 6A.

Although the foregoing invention has been described with reference tocertain exemplary embodiments, other embodiments will become apparent inview of this disclosure. Therefore, the described embodiments are to beconsidered only as illustrative and not restrictive. The scope of thepresent invention, therefore, is indicated by the appended claims andtheir combination in whole or in part rather than by the foregoingdescription. All changes thereto would come within the meaning and rangeof the equivalence of the claims are to be embraced within their scope.

1. A method of forming a memory cell, comprising: forming a trenchwithin a substrate, whereby a ridge in the substrate is definedproximate to the trench; filling the trench with an insulating material;forming a semiconductor pillar over the ridge; forming a source regionwithin the ridge adjacent to the semiconductor pillar; recessing theinsulating material in the trench below a top point of the ridge; anddepositing a gate material after recessing the insulating material,wherein the gate material extends lower than the top point of the ridge.2. The method of claim 1, further comprising forming a digit line abovethe pillar after depositing the gate material.
 3. The method of claim 1,wherein a midgap potential proximate to the gate material is reduced byat least 75% compared to an unrecessed gate material.
 4. The method ofclaim 3, wherein the midgap potential proximate to the gate material isreduced by at least 85% compared to the unrecessed gate material.
 5. Themethod of claim 1, further comprising extending the source region tooverlap with the semiconductor pillar.
 6. The method of claim 1, furthercomprising forming a capacitor over the semiconductor pillar afterdepositing the gate material, wherein forming the capacitor over thesemiconductor pillar comprises: forming a conductive plug over thesemiconductor pillar; and forming a container capacitor contacting theconductive plug.
 7. The method of claim 1, wherein filling the trenchcomprises depositing an oxide.
 8. The method of claim 1, wherein formingthe semiconductor pillar comprises forming a first mask having aplurality of lines over the substrate; forming a second mask between thelines of the first mask wherein the first mask and the second mask forman exposed substrate island; providing a spacer layer over the firstmask and lining the second mask and the exposed substrate islands;punching a contact through the spacer layer to the exposed substrateisland; and growing epitaxial silicon within the exposed substrateisland after punching the contact.
 9. The method of claim 1, whereinforming the semiconductor pillar comprises forming a rectangular pillar.10. The method of claim 1, wherein forming the semiconductor pillarcomprises forming a cylindrical pillar.
 11. The method of claim 1,wherein recessing the insulating material below a top point of the ridgecomprises recessing the insulating material in the trench between about0 Å and 1500 Å.
 12. The method of claim 11, wherein recessing theinsulating material below a top point of the ridge comprises recessingthe insulating material in the trench between about 5003 Å and 1000 Å.13. The method of claim 1, further comprising forming a gate dielectricsurrounding the semiconductor pillar prior to depositing the gatematerial.
 14. The method of claim 13, wherein forming the gatedielectric comprises thermally growing an oxide over the semiconductorpillar.
 15. The method of claim 13, wherein forming the gate dielectriccomprises forming an oxide with a thickness of between about 10 Å andabout 100 Å.
 16. The method of claim 1, further comprising forming adigit line after depositing the gate material.
 17. The method of claim16, further comprising depositing a digit line plug.
 18. The method ofclaim 1, further comprising doping an upper portion of the ridge betweenthe source region and a portion of the ridge below the semiconductorpillar.
 19. A method of forming a memory array comprising: defining aplurality of parallel trenches and ridges in a substrate; filling thetrenches with an isolation material; forming a plurality of pillars overthe ridges; recessing the isolation material in the trench after formingthe pillars; and depositing a plurality of word lines over the isolationmaterial and the ridges and surrounding a row of pillars after formingthe pillars, wherein the word lines descend substantially into thetrenches.
 20. The method of claim 19, further comprising forming a cellcapacitor over each pillar.
 21. The method of claim 19, furthercomprising forming a plurality of source regions within the ridges. 22.The method of claim 19, further comprising forming a plurality of drainregions on the pillars.
 23. The method of claim 19, wherein forming thepillars comprises using a selective epitaxial growth step.